The present invention generally relates to the field of communications systems and, more particularly, to the field of radio communications systems which measure transmission quality.
Commercial communication systems and, in particular, cellular radiotelephone systems have experienced explosive growth in the past decade. This growth is due, at least in part, to the improvement in the number and quality of services provided by radiocommunication systems. For example, early systems were designed primarily to support voice communications. However, cellular radiocommunication systems now provide many additional services including, for example, paging, messaging and data communications (e.g., to support Internet communication). Some of these new services make higher throughputs, than needed for voice communication, very desirable.
In order to provide these various communication services, a corresponding minimum user bit rate is required. For example, for voice and/or data services, user bit rate corresponds to voice quality and/or data throughput, with a higher user bit rate producing better voice quality and/or higher data throughput. The total user bit rate is determined by a selected combination of techniques, e.g., speech coding, channel coding, modulation scheme, and the air interface resources allocated to the connection, i.e., for a TDMA system, the number of assignable time slots, for a CDMA system the number of spreading codes.
Consider the impact of different modulation schemes on the user bit rate. Conventionally, different digital communication systems have used a variety of linear and non-linear modulation schemes to communicate voice or data information. These modulation schemes include, for example, Gaussian Minimum Shift Keying (GMSK), Quadrature Phase Shift Keying (QPSK), 8-ary Phase Shift Keying (8PSK), Quadrature Amplitude Modulation (QAM), etc. Typically, each communication system operates using a single modulation scheme for transmission of information under all conditions. For example, ETSI originally specified the GSM standard to communicate control, voice and data information over links using a GMSK modulation scheme to provide transmission of information.
Depending on the modulation scheme used by a particular system, the throughput of a packet transmission scheme deteriorates differently as C/I levels decrease. For example, modulation schemes may use a different number of values or levels to represent information symbols. The signal set, i.e., amplitude coefficients, associated with QPSK, an exemplary lower level modulation (LLM) scheme, are illustrated in FIG. 1(a). By way of comparison, 16QAM is a higher level modulation (HLM) scheme having the signal set depicted in FIG. 1(b).
As can be seen in FIGS. 1(a) and 1(b), the minimum Euclidean distance between the coefficients in the LLM scheme is greater than the minimum Euclidean distance between coefficients in the HLM scheme for the same average signal power, which makes it easier for receive signal processing to distinguish between modulation changes in the LLM scheme. Thus, LLM schemes are more robust with respect to noise and interference, i.e., require a lower carrier-to-interference (C/I) level to achieve acceptable received link quality. HLM schemes, on the other hand, provide greater user bit rates, e.g., 16QAM provides twice the user bit rate of QPSK, but require higher C/I levels.
More recently, however, dynamic adaptation of the modulation used for transmission in radiocommunication systems types has been considered as an alternative that takes advantage of the strengths of individual modulation schemes to provide greater user bit rates and/or increased resistance to noise and interference. An example of a communication system employing multiple modulation schemes is found in U.S. Pat. No. 5,577,087. Therein, a technique for switching between 16QAM and QPSK is described. The decision to switch between modulation types is made based on quality measurements.
In addition to modulation schemes, digital communication systems also employ various techniques to handle erroneously received information. Generally speaking, these techniques include those which aid a receiver to correct the erroneously received information, e.g., forward error correction (FEC) techniques, and those which enable the erroneously received information to be retransmitted to the receiver, e.g., automatic retransmission request (ARQ) techniques. FEC techniques include, for example, convolutional or block coding of the data prior to modulation. FEC coding involves representing a certain number of data bits using a certain number of code bits. Thus, it is common to refer to convolutional codes by their code rates, e.g., xc2xd and ⅓, wherein the lower code rates provide greater error protection but lower user bit rates for a given channel bit rate. By adjusting the coding rate, the effective data throughput in a radiocommunication system can also be adjusted. Thus it can be seen that a number of techniques are contemplated for implementing variable data rates transmission in radiocommunication systems.
In standard remote terminals in use today, there is normally an indication of the received signal strength provided on the terminal""s display. However, this indicator only provides a very rough estimation of system quality available to the user. Further, for a data user, it is impossible to predict the achievable bit rate or throughput from the signal strength indication on conventional terminals. To obtain an accurate prediction of transmission throughput capabilities, factors such as: downlink interference from other cells on channels assigned to the current cell; carrier to interference ratio (C/I); bit error rate; block error rate; and time dispersion, along with received signal power should be taken into account. Two other factors which affect transmission throughput and should be accounted for are: support for multi-slot operation and support for different coding/modulation schemes.
Accordingly, it would be desirable to provide remote stations with a throughput indication, so that the users can adapt their interactions with the system accordingly.
The present invention provides a user with information regarding the throughput that he or she can expect to achieve, if the user were to initiate a connection in his present location. For example, the system can provide an indication of the maximal bit rate capabilities that are anticipated for a data connection with the system given, for example, the remote station""s current location and both the mobile station and the base station""s capabilities.
According to one exemplary embodiment of the present invention there is provided a method for indicating a predicted transmission throughput at a mobile station, comprising the steps of: measuring link quality of at least one channel; estimating the predicted transmission quality based on, at least one of, the mobile station""s capability and said link quality; and outputting the predicted transmission quality at the mobile station.
According to another embodiment of the invention there is provided a method for indicating a maximal transmission quality available within a cell, comprising the steps of: receiving at a mobile station, a message from a base station indicating a maximal base station transmission quality capability; determining the maximal transmission quality of a connection based on said maximal base station transmission quality capability and a maximal mobile station transmission quality capability; and outputting the maximal transmission quality at the mobile station.
According to another embodiment of the invention there is provided a method for indicating at the mobile station, both the maximal transmission quality which is possible within the current cell and the predicted transmission quality the user would achieve if a session were initiated at that time. By providing the user with both the maximal bit rate and the predicted bit rate, the user can compare the two indicators and make an informed decision as to whether or not he should move prior to initiating a connection.